3Com 10014298 manual Neighbor discovery mechanism is the same as that of PIM-DM

136CHAPTER 6: MULTICAST PROTOCOL

[SW7750-vlan-interface12]pim dm

Configuring PIM-SMPIM-SM (Protocol Independent Multicast, Sparse Mode) belongs to sparse mode multicast routing protocols. PIM-SM is mainly applicable to large-scale networks with broad scope and few group members.

Different from the flood & prune principle of the dense mode, PIM-SM assumes that all hosts do not need to receive multicast packets, unless clear request is put forward.

PIM-SM uses the RP (Rendezvous Point) and the BSR (Bootstrap Router) to advertise multicast information to all PIM-SM routers and uses the join/prune information of the router to build the RP-rooted shared tree (RPT). This helps to reduce the bandwidth occupied by data packets and control packets, and reduces the process overhead of the router. Multicast data flows along the shared tree to the network segments. When data traffic is sufficient, the multicast data flow switches over to the SPT (Shortest Path Tree) rooted on the source. This reduces network delay. To perform the RPF check, PIM-SM does not depend on the specified unicast routing protocol but uses the present unicast routing table.

Running PIM-SM, you would need to configure candidate RPs and BSRs. The BSR is responsible for collecting the information from the candidate RP and advertising the information.

Configuring PIM-SM is described in the following sections:

■PIM-SM Operating Principles

■Preparing to Configure PIM-SM

■Configuring PIM-SM

PIM-SM Operating The PIM-SM working process is as follows: neighbor discovery, building the Principles RP-rooted shared tree (RPT), multicast source registration and SPT switchover etc.

The neighbor discovery mechanism is the same as that of PIM-DM.

Build the RP shared tree (RPT)

When hosts join a multicast group G, the leaf routers send IGMP messages to learn the receivers of the multicast group G. The leaf routers calculate the corresponding rendezvous point (RP) for multicast group G, and then send join messages to the node of a higher level toward the rendezvous point (RP). Each router along the path, between the leaf routers and the RP, will generate (*, G) entries in the forwarding table, indicating that all packets sent to multicast group G are applicable. When the RP receives packets sent to multicast group G, the packets will be sent to leaf routers along the path built and then reach the hosts. In this way, an RP-rooted tree (RPT) is built as shown in Figure 34.